C++相关闲碎记录(17)-编程知识

1、IO操作

（1）class及其层次体系

（2）全局性stream对象

（3）用来处理stream状态的成员函数

前四个成员函数可以设置stream状态并返回一个bool值，注意fail()返回是failbit或者badbit两者中是否任一个设置，如果调用不带参数的clear()，所有的error flag均会被清除。

下面这个例子用于检查failbit是否设置，若设置则清除。

if(strm.rdstate() & std::ios::failbit){std::cout << "failbit was set" << std::endl;strm.clear(strm.rdstate() & ~std::ios::failbit);
}

（4）stream异常

下面的例子要求所stream对所有的flag均抛出异常：

strm.exceptions(std::ios::eofbit | std::ios::failbit | std::ios::badbit);

但是如果传入0或者goodbit，就不会引发异常。

strm.exceptions(std::ios::goodbit);

异常抛出的时机是在“程序调用clear()或setstate()之后”又设置了某些flag之际，如果某个标志已被设置但未被清除，也会抛出异常。

下面的例子从输入中读取浮点数，直到end-of-file为止，返回总和。

#include <iostream>
#include <exception>
#include <cstdlib>namespace MyLib {double readAndProcessSum (std::istream&);
}int main()
{using namespace std;double sum;try {sum = MyLib::readAndProcessSum(cin);}catch (const ios::failure& error) {cerr << "I/O exception: " << error.what() << endl;return EXIT_FAILURE;}catch (const exception& error) {cerr << "standard exception: " << error.what() << endl;return EXIT_FAILURE;}catch (...) {cerr << "unknown exception" << endl;return EXIT_FAILURE;}// print sumcout << "sum: " << sum << endl;
}#include <istream>namespace MyLib {double readAndProcessSum (std::istream& strm){using std::ios;double value, sum;// save current state of exception flagsios::iostate oldExceptions = strm.exceptions();// let failbit and badbit throw exceptions// - NOTE: failbit is also set at end-of-filestrm.exceptions (ios::failbit | ios::badbit);try {// while stream is OK// - read value and add it to sumsum = 0;while (strm >> value) {sum += value;}}catch (...) {// if exception not caused by end-of-file// - restore old state of exception flags// - rethrow exceptionif (!strm.eof()) {strm.exceptions(oldExceptions);  // restore exception flagsthrow;                           // rethrow}}// restore old state of exception flagsstrm.exceptions (oldExceptions);// return sumreturn sum;}
}

（5）读写字符的成员函数

（6）输出控制manipulator

（7）用户自定义操控器

#include <istream>
#include <limits>template <typename charT, typename traits>
inline
std::basic_istream<charT,traits>&
ignoreLine (std::basic_istream<charT,traits>& strm)
{// skip until end-of-linestrm.ignore(std::numeric_limits<std::streamsize>::max(),strm.widen('\n'));// return stream for concatenationreturn strm;
}

这个控制器用来忽略一行，如果要忽略多行，就调用多次

std::cin >> ignoreLine >> ignoreLine;

函数ignore(max, c)会略去input stream中的字符c之前的所有字符，如果前面的字符多余max个，就略去max个，如果先遇到stream结尾，就全部忽略。

#include <iostream>
#include "ignore1.hpp"int main()
{int i;std::cout << "read int and ignore rest of the line" << std::endl;std::cin >> i;// ignore the rest of the linestd::cin >> ignoreLine;std::cout << "int: " << i << std::endl;std::cout << "read int and ignore two lines" << std::endl;std::cin >> i;// ignore two linesstd::cin >> ignoreLine >> ignoreLine;std::cout << "int: " << i << std::endl;
}

#include <istream>
#include <limits>class ignoreLine
{private:int num;public:explicit ignoreLine (int n=1) : num(n) {}template <typename charT, typename traits>friend std::basic_istream<charT,traits>&operator>> (std::basic_istream<charT,traits>& strm,const ignoreLine& ign){// skip until end-of-line num timesfor (int i=0; i<ign.num; ++i) {strm.ignore(std::numeric_limits<std::streamsize>::max(),strm.widen('\n'));}// return stream for concatenationreturn strm;}
};

#include <iostream>
#include "ignore2.hpp"int main()
{int i;std::cout << "read int and ignore rest of the line" << std::endl;std::cin >> i;// ignore the rest of the linestd::cin >> ignoreLine();std::cout << "int: " << i << std::endl;std::cout << "read int and ignore two lines" << std::endl;std::cin >> i;// ignore two linesstd::cin >> ignoreLine(2);std::cout << "int: " << i << std::endl;std::cout << "read int: " << std::endl;std::cin >> i;std::cout << "int: " << i << std::endl;
}

（8）format flag格式标志

//set flags showpos and uppercase
std::cout.setf(std::ios::showpos | std::ios::uppercase);
//set only the flag hex in the group basefield
std::cout.setf(std::ios::hex, std::ios::basefield);
//clear the flag uppercase
std::cout.unsetf(std::ios::uppercase);

using std::ios;
using std::cout;
//save current format flags
ios::fmtflags oldFlags = cout.flags();//do some changes
cout.setf(ios::showpos | ios::showbase | ios::uppercase);
cout.setf(ios::internal, ios::adjustfield);
cout << std::hex << x << std::endl;
cout.flags(oldFlags);

（9）文件读写

（10）文件flag

//seek to the beginning of the file
file.seek(0, std::ios::beg);
//seek 20 characters forward
file.seek(20, std::ios::cur);
//seek 10 characters before the end
file.seek(-10, std::ios::end);

（11）重定向

#include <iostream>
#include <fstream>
#include <memory>
using namespace std;void redirect(ostream&);int main()
{cout << "the first row" << endl; //输出控制台redirect(cout);  //重定向到文件中，执行完毕之后，又重新指向控制台cout << "the last row" << endl;//指向控制台
}void redirect (ostream& strm)
{// save output buffer of the stream// - use unique pointer with deleter that ensures to restore//     the original output buffer at the end of the function// 定义了一个删除器auto del = [&](streambuf* p) {strm.rdbuf(p);};// 使用智能指针的目的时在退出函数时，还原ostreamunique_ptr<streambuf,decltype(del)> origBuffer(strm.rdbuf(),del);// redirect ouput into the file redirect.txtofstream file("redirect.txt");// strm指向file缓冲区strm.rdbuf (file.rdbuf());// 此两字符都会写入到文件中file << "one row for the file" << endl;strm << "one row for the stream" << endl;// 程序结束时，调用智能指针的删除器，将输出重新指向p，而这个p就是strm.rdbuf()
} //

（12）可读写的stream

定义一个file stream缓冲区，并将它安装在两个stream对象上，

std::filebuf buffer;
std::ostream out(&buffer);
std::istream in(&buffer);
buffer.open("example.txt", std::ios::in | std::ios::out);

#include <iostream>
#include <fstream>
using namespace std;int main()
{// open file "example.dat" for reading and writingfilebuf buffer;ostream output(&buffer);istream input(&buffer);buffer.open ("example.dat", ios::in | ios::out | ios::trunc);for (int i=1; i<=4; i++) {// write one lineoutput << i << ". line" << endl;// print all file contentsinput.seekg(0);          // seek to the beginningchar c;while (input.get(c)) {cout.put(c);}cout << endl;input.clear();           // clear  eofbit and  failbit}
}
输出：
1. line1. line
2. line1. line
2. line
3. line1. line
2. line
3. line
4. line

（13）stream 缓冲区接口

函数pubseekoff()和pubseekpos()控制读写动作的当前位置，究竟是控制读或者写，取决于最后实参，其类型为ios_base::openmode，如果没有特别指定，实参默认值为ios_base::in|ios_base::out，一旦设置ios_base::in，读的位置就会跟着改变，一旦设置ios_base::out，写的位置也会跟着变化，函数pubseekpos()会把stream当前位置移至第一实参指示的绝对位置上，函数pubseekoff()则把stream当前位置移至某个相对位置，偏移量由第一实参决定，起始位置由第二实参决定，可以是ios_base::cur, ios_base::beg, ios_base::end。两个函数都返回stream所在的位置或者一个无效的位置，将函数的结果拿来和对象pos_type(off_type(-1))比较(pos_type和off_type是处理stream位置时所用的类型），如果希望获取stream当前位置，可以使用pubseekoff(): sbuf.pubseekoff(0, std::ios:cur)。

（14）output stream 缓冲区的iterator

使用ostreambuf_iterator将一个字符串写入stream缓冲区内：

std::ostreambuf_iterator<char> bufWriter(std::cout);
std::string hello("hello, world\n");
std::copy(hello.begin(), hello.end(), bufWriter);

（15）input stream 缓冲区的iterator

将输入缓冲区的字符输出：

#include <iostream>
#include <iterator>
using namespace std;int main()
{// input stream buffer iterator for cinistreambuf_iterator<char> inpos(cin);// end-of-stream iteratoristreambuf_iterator<char> endpos;// output stream buffer iterator for coutostreambuf_iterator<char> outpos(cout);// while input iterator is validwhile (inpos != endpos) {*outpos = *inpos;    // assign its value to the output iterator++inpos;++outpos;}
}